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-   -   Unsteady Combustion (http://www.cfd-online.com/Forums/fluent/105233-unsteady-combustion.html)

NormalVector July 25, 2012 18:36

Unsteady Combustion
 
Hello,

I'm having a bit of trouble successfully running a hydrogen-air transient combustion case. The basic geometry is a cylinder with a smaller, short concentric cylinder. Oxygen is introduced at the larger cylinder's cross-section while hydrogen is introduced inside of the smaller cylinder and free to flow out. Both are mass flow inlets. I am able to run a steady state simulation and achieve combustion but when I switch over to a transient regime, the mixture never ignites and remains at the species inlet temperature.

I've tried increasing the temperature of both species as to induce ignition but no luck. I've also tried timesteps ranging from 1e-05 to 1e-03 and no luck.


Some solver specifics:
  • Species transport and reactions combustion model (Volumetric, Finite Rate/Eddy Dissipation
  • k-e realizable turbulence model
  • I've used both PISO and SIMPLE pressure-velocity coupling schemes
  • All spatial discretization entries are first order upwind
Any help would be greatly appreciated. As I said, steady state works fine with the same solver settings (if applicable). Switching over to transient results in no ignition.

eng_s_sadeghi July 25, 2012 23:05

Unsteady combustion is really awful. Please use spark model for ignition on a point and also try to patch a small amount of species (reactants and products) around the point. I always use it.

NormalVector July 26, 2012 01:53

Quote:

Originally Posted by eng_s_sadeghi (Post 373601)
Unsteady combustion is really awful. Please use spark model for ignition on a point and also try to patch a small amount of species (reactants and products) around the point. I always use it.

Thanks, at least I know I'm not the only one who has had trouble with transient combustion. Is this a known problem in FLUENT? I tried a spark earlier today but that didn't do anything, it was merely an increase in temperature. Are there any spark parameters you recommend (spark energy, diameter, etc)?

eng_s_sadeghi July 26, 2012 03:05

You should increase the spark energy to get a temperature of about 2000K at the point. Also there should be at least a small amount of both reactants and products. Don't forget products should also be around the spark point.

NormalVector July 26, 2012 16:33

Alright thanks, I'll give it a try.

NormalVector July 30, 2012 18:17

Quote:

Originally Posted by eng_s_sadeghi (Post 373627)
You should increase the spark energy to get a temperature of about 2000K at the point. Also there should be at least a small amount of both reactants and products. Don't forget products should also be around the spark point.

Thanks for the help. I was able to get my simulation to combust. Originally, I was using the hydrogen-air mixture template and only included oxygen and hydrogen in the computational domain. When I allowed water to flow into the domain along with the oxygen, the mixture successfully combusted. This was not the case for a steady simulation: the combustion occurred without water present.

haibai April 23, 2014 04:35

fluent spark ignited
 
hi NormalVector,i have met the same problem to use the spark ignited,
I am able to run a steady state simulation and achieve combustion but when I switch over to a transient regime, the mixture never ignites and remains at the species inlet temperature,
i see you have solved the problem,so i am very desired if you could help me
thanks very much

NormalVector April 23, 2014 12:52

See my post above, try adding a small amount of products into your domain.

haibai April 23, 2014 13:26

Quote:

Originally Posted by NormalVector (Post 487766)
See my post above, try adding a small amount of products into your domain.

thanks for your help,i have do it as you said above, rigion a zone around the spark point,and add 0.01productsand some reactants,but it doesnot work,are there other parameters should notice??? desire for your reply

NormalVector April 23, 2014 13:58

The only other thing I would suggest is to initialize your entire domain with a small amount of product. That fixed my problem so if that does not work then I don't know what is wrong with your simulation.

haibai April 24, 2014 03:23

Quote:

Originally Posted by NormalVector (Post 487777)
The only other thing I would suggest is to initialize your entire domain with a small amount of product. That fixed my problem so if that does not work then I don't know what is wrong with your simulation.

thank you i will give it try again
i give the spark energy 2500j and my inlet is 0.05ch4 0.2 o2 ;
i wonder if my inlet boundary conditions set is suited
i am very glad to see you reply


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